1. Technical Field of the Invention
The present invention relates, generally, to alkali silicate glasses which are used as the basic glasses for the manufacture of selective filter glasses. More particularly, the present invention relates to the manufacture of selective filter glasses, colored with Co.sup.2+ or with Co.sup.2+ and Ni.sup.2+, which have transmission ranges primarily in the ultraviolet and visible spectral region between 280 and 500 nanometers (nm). These ion-colored glasses are imbued with the characteristic absorption bands of Co.sup.2+ and Co.sup.2+ together with Ni.sup.2+ and, therefore, depending upon the layer thickness, produce selective bell-shaped transmission ranges, each surrounded by two stop ranges.
The most important characteristics of these so-called band pass filters, when the layer thickness is known, are the maximum of spectral transmission in the transmission band, T.sub.max and the half-value wavelengths .lambda.'1/2 and .lambda."1/2 with a spectral transmission of T.sub.max /2. The mid-point between the half-value wavelengths .lambda.'1/2 and .lambda."1/2 is called the mean wavelength .lambda..sub.m ; the distance from .lambda.'1/2 to .lambda."1/2 is known as the half-value width (HW). (See FIG. 1.)
In research, engineering and medicine, these glasses are used, for example, as conversion filters in a narrow spectral region and, more particularly, are used in the filtering of UV light sources in the emission range from 200 to 1100 nm, with particular significance as pass filters in the ultraviolet spectral region from 250 to 400 nm and as stop filters in the spectral region of approximately 420 to 680 nm. The glasses can readily have compositions that assure high chemical resistance to acids, alkalis and water.
2. Description of the Prior Art
Alkali silicate glasses colored with Co.sup.2+ or with Co.sup.2+ and Ni.sup.2+ are available from the major manufacture of optical filter glass and, therefore, known to the art, e.g. U.S. Pat. No. 3,902,881. Examples include BG 3, FG 3, UG 1 (Schott catalogue), 1-61, 7-51 (Corning catalogue), BD 37-93 (VEB Jena catalogue) and others. A drawback experienced with these glasses is that if they are exposed to ultraviolet radiation for a relatively long duration, the spectral transmission of these glasses can change, predominantly in the ultraviolet spectral range. The absorption increases up to a certain saturation value. This undesirable phenomenon is called solarization. The aforementioned saturation value is attained at varying degrees of quickness, depending upon the duration and intensity of the irradiation and on the distance of the filter glass from the light source. The glass "ages" and then
is only conditionally usable for its intended purpose.
For ascertaining the solarization properties of filter glasses, the following testing procedure is typical and generally applicable:
thickness of sample: 1 mm PA0 light source: low-pressure mercury lamp (maximum emission at 254 nm), type: "Sylvania Germicidal," G 15 T 8, sold by Philips (emission spectrum, see FIG. 2) PA0 filtering of source: none PA0 exposure time: 75 hours PA0 sample-to-source distance:140 nm PA0 measured variable: .DELTA..tau. (transmission before and after UV exposure)
In FIGS. 3 and 4, transmission curves are plotted, from 200 to 850 nm for both before and after the above-described solarization testing procedure, taking a commercially available blue alkali silicate glass as an example (BG 3 made by Schott Glaswerke, Mainz, Federal Republic of Germany). The curves demonstrate the very severe change in filter properties in the transmission band from 260 to 520 nm.